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NAME

       log1p, log1pf, log1pl - compute a natural logarithm

SYNOPSIS

       #include <math.h>

       double log1p(double x);
       float log1pf(float x);
       long double log1pl(long double x);

DESCRIPTION

       These functions shall compute log_e(1.0 + x).

       An  application  wishing to check for error situations should set errno
       to zero and  call  feclearexcept(FE_ALL_EXCEPT)  before  calling  these
       functions.   On return, if errno is non-zero or fetestexcept(FE_INVALID
       | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error  has
       occurred.

RETURN VALUE

       Upon  successful  completion,  these functions shall return the natural
       logarithm of 1.0 + x.

       If x is -1, a  pole  error  shall  occur  and  log1p(),  log1pf(),  and
       log1pl()   shall   return   -HUGE_VAL,   -HUGE_VALF,   and  -HUGE_VALL,
       respectively.

       For finite values of x that are less than -1,    or if x  is  -Inf,   a
       domain  error  shall occur, and    either a NaN (if supported), or   an
       implementation-defined value shall be returned.

       If x is NaN, a NaN shall be returned.

       If x is ±0, or +Inf, x shall be returned.

       If x is subnormal, a range error may occur and x should be returned.

ERRORS

       These functions shall fail if:

       Domain Error
              The finite value of x is less than -1,    or x is -Inf.

       If the integer expression (math_errhandling & MATH_ERRNO) is  non-zero,
       then   errno  shall  be  set  to  [EDOM].  If  the  integer  expression
       (math_errhandling &  MATH_ERREXCEPT)  is  non-zero,  then  the  invalid
       floating-point exception shall be raised.

       Pole Error
              The value of x is -1.

       If  the integer expression (math_errhandling & MATH_ERRNO) is non-zero,
       then errno  shall  be  set  to  [ERANGE].  If  the  integer  expression
       (math_errhandling  &  MATH_ERREXCEPT)  is non-zero, then the divide-by-
       zero floating-point exception shall be raised.

       These functions may fail if:

       Range Error
              The value of x is subnormal.

       If the integer expression (math_errhandling & MATH_ERRNO) is  non-zero,
       then  errno  shall  be  set  to  [ERANGE].  If  the  integer expression
       (math_errhandling & MATH_ERREXCEPT) is  non-zero,  then  the  underflow
       floating-point exception shall be raised.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       On   error,   the   expressions  (math_errhandling  &  MATH_ERRNO)  and
       (math_errhandling & MATH_ERREXCEPT) are independent of each other,  but
       at least one of them must be non-zero.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       feclearexcept()  , fetestexcept() , log() , the Base Definitions volume
       of IEEE Std 1003.1-2001, Section 4.18, Treatment  of  Error  Conditions
       for Mathematical Functions, <math.h>

COPYRIGHT

       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003  by  the  Institute  of
       Electrical  and  Electronics  Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE and
       The  Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be  obtained  online
       at http://www.opengroup.org/unix/online.html .